A large variety of organic corrosion inhibiting compounds such as N-nitrosamine and/or benzol have been embedded or otherwise confined in packaging materials, more particularly in sheet stock made from paper or other cellulosic materials, or from a polymer of an olefin containing from 2 to 8 carbon atoms. Such compounds function as VCIs without directly contacting the surface of the metal being protected, except to the extent that particles of a VCI at the surface of a wrap or sheet of packaging material contact the surface of the object packaged. A VCI is a “noncontact inhibitor”. Therefore, the effectiveness of a VCI is necessarily predicated upon its ability to provide protection against corrosion without contacting surfaces to be protected, for example, even if the object to be protected is suspended in a sealed space provided by packaging material containing the VCI, as demonstrated by a test described herebelow.
Unlike VCIs, rust-inhibiting inorganic alkaline salts have long been used to combat corrosion, as for example in U.S. Pat. No. 3,110,684 which discloses a combination of from 30% to 50% by weight of the alkaline salt, with 30% to 50% by weight of a sequestering or chelating agent, and 10% to 30% by weight of a preservative agent. Examples of the alkaline salt include sodium silicate, sodium nitrite, sodium nitrate, sodium chromate, sodium chlorate, or sodium carbonate, or potassium silicate, potassium nitrite, potassium nitrate, etc. or lithium silicate, etc.; examples of a sequestering or chelating agent include a sodium salt of phosphoric acid, such as tetrasodium pyrophosphate, sodium citrate or sodium gluconate, a potassium salt of phosphoric acid, potassium citrate, etc., or an ammonium salt of phosphoric acid, ammonium citrate, etc., citric acid or gluconic acid or ethylene diamine tetra acetic acid; examples of a sequestering agent include sodium benzoate, ammonium benzoate or lithium benzoate.
U.S. Pat. No. 3,304,267 teaches a combination of from 75-85% by weight of a rust inhibiting alkaline salt, 5-15% by weight of sodium metasilicate, and from 5 to 15% by weight of a non-ionic organic dispersant compressed and molded into a solid which operates as a corrosion inhibitor only when water is present. Hungarian Patent HU 48857 teaches using an aqueous paste of perlite, bentonite containing sodium silicate and/or sodium phosphate as active ingredients, which paste is plastic enough to be pressed into holes in steel used to reinforce concrete. However, such inorganic materials, with the exception of sodium nitrite have not been suggested for use as VCIs because of their much lower vapor pressure compared to that exerted by corrosion inhibiting organic compounds. Additional problems with respect to using other inorganic compounds include the difficulty of grinding or otherwise comminuting them to a sufficiently small particle size so as to be rendered essentially invisible when dispersed in transparent film; of dispersing them uniformly in the film because of their proclivity to clump; and, of extruding film with surfaces smooth to the touch, because of the proclivity of the particles to come to the surfaces of the film.
With the foregoing in mind, U.S. Pat. No. 4,290,912 issued to Boerwinkle et al, about two decades ago, disclosed that an inorganic nitrite, e.g. potassium nitrite, sodium nitrite and calcium nitrite, in combination with a 2,4,6-trisubstituted phenol provided a VCI combination in a lower PO polymer. Neither potassium nitrite nor calcium nitrite is a usable VCI since they are too hygroscopic. A specific '912 combination comprised about equal parts (1.485 phr each) by weight of sodium nitrite and a 2,4,6-tri-substituted phenol containing 9 to 24 carbon atoms, specifically 2,6-di-tert-butyl-4-methyl phenol, along with small amounts of one or more inert ingredients such as fumed silica and oleyl alcohol which are known to possess no anti-corrosive properties. As of this time, sodium nitrite is the only usable VCI found to be effective with the 2,4,6-tri-substituted phenol. Effective-ness of the '912 film was unconcerned with the primary particle size of the sodium nitrite because the '912 patent did not address the problems (i) of embrittlement, (ii) of maintaining transparency and/or uniform thickness of extruded film having smooth surfaces, or (iii) of protection against sulfur dioxide, or the ability to tailor the thickness of film to provide protection for a predetermined period under expected unfavorable storage conditions.
The newly discovered effectiveness of sodium silicate and/or zinc oxide in combination with the '912 VCI combination of active ingredients was unexpected because there is no reason to believe that either the stable essentially anhydrous silicate or amphoteric zinc oxide, when combined with a non-hydrolyzable polymer having a WVTR at least as high as that of LDPE, might interact with an acid gas such as sulfur dioxide, present in an amount in the range from about 10 ppm to 100 ppm, to function predominantly as an interceptor; and, in combination with sodium nitrite and the 2,4,6-tri-substituted phenol function as a VCI; further, neither the silicate nor the zinc oxide, by itself, has any discernible anti-corrosive activity as a VCI; still further, the WVTR (water vapor transmission rate) of the novel film which contains the interceptor, whether zinc oxide or silicate of sodium, or both, in addition to the '912 ingredients, after about 24 hr, is essentially the same as that of the plain PE film with no VCI ingredients (as shown in Table I below), or of the '912 film.
Since a lower WVTR with polymer containing the interceptor, would appear to provide a theoretical basis upon which to expect better corrosion protection, no decrease in the WVTR provided a reasonable basis not to use one or more of the interceptors as an essential ingredient of the VCI combination. By “essentially non-hydrolyzable” is meant that the polymer is not hydrolyzed more than 5% under conditions at which an object to be protected is stored while exposed to the VCI ingredients in a sealed environment.